OK, makes more sense, but you still didn't say whether you have SIX total speakers. Meaning front-right, front-left, rear-right, rear-left, center, and sub.
I'm just guessing you DO have 5 speakers (front, back and center), and ALL 5 of those work from your testing?
And also your SUB works too, as in, makes sound?
So in other, other words
All SIX of your speakers work, but NOT from the center speaker port?
Ok, then answer this, how does the sub connect to the PC? Is it through one digital cable? Or multiple cables?
If it's digital, single cable for all channels, then there is no good way to test really, I'd probably say the output on the sub has gone bad. Because it would be very hard for a sound card to mux up just one PART of a multi-channel signal over a single cable.
My guess is the port on the sub is bad, I've seen it before.
But really you can only test 100% with another set of speakers. Or test your entire set of speakers on someone else's 5.1 channel output.
Sorry about the confusion, just trying to get a mental picture here.